Sensitized blue print paper and coating therefor



Patented Sept. 21, 1937;

snnsrrrznn ,nwn mum rsrna AND' a coamo macros Alger P. Reynolds, NorthBeverly, Mash, asslgnor to'Spaulding-Moss Company, Boston, Mass, acorporation of Massachusetts No Drawing. "Application February 25, 1936,I Serial No. 65,861

1. Claims.

The present invention relates to the art of making blue prints" bycoating a suitabiebase (which base for the sake of convenience ishereinafter referred to as "paper", butwhich expression is intended tocover any suitable base which may be used, such as paper, cloth, etc.)with a solution which is sensitive to light so that when the sensitizedpaper is subsequently exposed to light and then subjected to a suitabledeveloping bath a permanent blue color is obtained wherever the lightreached the sensitized paper during exposure.

The primary object of my invention is toprovides. more highly sensitivecoating for blue print paperfwhich requires exposure to light a shorterperiod of time during printing, thereby providing a faster paper.

Another object 'is to provide such a coating which results in a deeperblue background in the exposed areas in the final print.

A further object is to eliminate fogging of the white unexposed surfacesor lines in the final print. I 1

Another object is to provide such an improved sensitized paper which maybe kept for longer periods of time in sensitized condition withoutdeterioration prior to exposure to light.

A' further object is to provide a coating solution which maybe appliedto the paper at room temperature, 1. e., without heating the solutionduring coating.

A further object is to provide a coating solution which when applied tothe paperscauses the paper size to swell or expand thereby permittingmore of the coating to be adsorbed by the paper and causing more of thesolution to be held thereon. J I

Other objects are to provide an improved blue print paper coating, andan improved coating solution. T I

In the prior art it has long been the established practice to'sensitizethe paper with a coating solution having as its essential elementsavwater solution of. a light-sensitive iron salt in theferric state,such as ferric ammonium oxalate, and a suitable ferricyanide, such aspotassium ferricysnide. When the coated paper has been dried it is fullysensitized and-ready for useand is the light-sensitive ferric salt to aferrous salt.'

Upon washing the exposed paper with water-the ferrous salt combineswith'the ferricyanide to form an insoluble blue compound, andtheunexposed and unreduced ferric salt which is found on the blue printpaper beneath the opaque lines of the diagram is dissolved paper leavinga white diagram upon a blue background. After being washed-with waterthe exposed paper is commonly washed in a bath containing an oxidizingagent, such as potassium bichromate, thereby bringing out in full theblue background and the paper is then again washed with water andsubsequently dried.- v 7 Either. partial or complete substitution of aferrocyanide for they ferricyanide in the abovereferred to prior artcoating bath has been suggested to thereby obtain a faster printingpaper which gives sharper white lines uponl'a deeper,

blue background; These" suggestions will-be found in the Britishpatent-to Holden No. 418,369 of 1934. In-the only suggestions of theuseof ferrocyanide found in the prior 'art, itba'sgbeen statedthat theentire coating solution, is a; water,

light-sensitiveferric'salt and ferrocyanide (and ferricyanide, if thelatter is also used) must not out of the blueprint 20 be acid, and thiscoating'solution as awholehas.

been'madeeither neutral or alkaline by the use I in thesolution'ofsuitable combinations of borax, ammonia, sodium hydroxide orpotassium oxalate, and the use of these chemicals has been understood bythe art to be buffering? the coating solution to-neutral or alkaline. II

When an alkali metal ferrocyanide, such as potassium', sodium orammonium ferrocyanide, is added to a water solution of a lightsen'sitive ferric salt; -suchf as ferric ammonium oxalate a bluesubstance is formed before exposure to light and after exposure-andwashing apapei coated with such a coating solution retains aconsiderable amount of the blue substance' upon the unent which preventsthe retention of this blue substance upon the unexposed areas thus per-;mitting it who washed completely off after print- .ing, leaving clearwhites The art has believed that the incorporation of any suitablebuffer in this ferrocyanide coating solution to thereby make thesolution as a whole neutral or alkaline would prevent thisfogging of thewhites.

I have found, however, that a much better ferrocyanide coating solutionis obtained when the coating solution as whole is acid and not when itis buffered to neutral or alkaline, and I have above, 1. e., neutral orI discovered that when oneor more of a certain class of chemicals, whichI define .hydrolyzable alkali metal compounds are bodied in the watersolution of a light-sensitiveferric salt and an alkali metalferrocyanide in such quantities that this'coating solution as a whole isacid, 1. e., with a pH value below 7," not onlyhthis tolling 0fthewhitel entirely eliminated but also the paper coated therewith printsconsiderably faster, with sharper white lines upon a deeper bluebackground, is easier to keep in sensitized condition, may be coatedwithout heating the coating solution above room temperature, and may becoated so that greater quantities of the solution are more readily andmore thoroughly adsorbed by the surface of the paper than by the use ofany neutral or alkaline ferrocyanide containing coating solutionsuggested by the prior art.

By the term "alkali metal in this specification and the annexed claims,I intend to include only potassium, sodium, cesium, rubidium, lithium,and ammonia, which latter substance although it is not, strictlyspeaking, a metal,- is commonly classed in the alkali metal group. Byalkaline hydrolyzable" compounds or salts wherever said term is used inthis specification and the annexed claims, I intend to include onlywater soluble salts or compounds, the water solutions of which have a-pHvalue'of greater than 7.

Thus'the class of chemicals referred to as alkaline hydrolyzable alkalimetal compounds includes chemicals such as trisodium phosphate, disodiumphosphate, trisodium arsenate, disodium arsenate, trisodium citrate,tripotassium citrate, ammonium sodium phosphate, sodium borophosphate,sodium glycerophosphate, magnesium ammonium citrate, sodiumpyrophosphate, sodium metasilicate, sodium sesquisilicate. sodiumcarbonate, sodium bicarbonate, sodium sesquicarbonatc, potassiumchromate, potassium tartrate, potassium ,borotartrate, sodiumtetraborate, potassium oxalate,, sodium hydroxide, potassium hydroxideand ammonium hydroxide. Other chemicals falling within this class willbe apparent to those skilled in the art. -It will be noted that ammoniumsodium phosphate, sodium borophosphate, sodium glycerophosphate,magnesium ammonium vcitrate, potassium borotartrate, sodiumsesquicarbonate and sodium .sesquisilicate of the above examples are socalled complex alkali metal compounds which hydrolyze alkaline. By theterm "alkali metal compounds I intend to include complex compounds. socalled, in which one-or more alkali metals are presentand whether or notsuch compounds contain in addition a metal other than an alkali metal.

Most 01' these alkaline hydrolyzable alkali metal compounds whenincorporated either separately or in combination with each other in ,awater solution of a light-sensitive iron salt in the ferric state, suchas ferric ammonium oxalate, and a suitable ferrocyanide, such aspotassium ierrocyanide (with or without a ferricyanide), produce acoating solution which has a pH value below 7 and is vtherefore acid.,However, the above-mentioned hydroxides and a very few of theabove-mentioned salts, such as sodium tetraborate or potassium oxalate,when used together in certain proportions and incorporated in the watersolution of the light-sensitive iron salt in the'ferric state and theferrocyanide (with or withouta ferricyanide), produce a coating solutionwhich is neutral or alkaline as a whole. Forinstance, the coatingsolution in the example given at page 2 of the Holden British patent hasa pH value of 7.2. In accordance with my invention the pH value of sucha coating solution must be reduced below '7, and this may be done byadding suitable proportions of weak inorganic acids,

such as. pyrophosphoric, orthophosphoric, boric, silicic, carbonic, orarsenic, or suitable propor-v tions of certain organic acids, such asoxalic, citric, tartaric, or malonic, or suitable proportions of a saltwhich hydrolyzes acid and is formed by the reaction between an alkalimetal and certain of said weak inorganic acids, such as'phosphoric, or.certain 'of said organic acids, such as oxalic. citric, or tartaric.Examples of such salts are monopotassium orthophosphate, monosodiumorthophosphate, sodium binoxalate, sodium bitartrate and sodium acidcitrate. The addition of suitable proportions of such acids or suchsalts which hydrolyze acid will make the coating solution as a wholeacid without at the same time resulting in the above-mentioned foggingoi' the whites.

- Of the above class which I have defined as alkaline hydrolyzablealkali metal compounds, I prefer to' use a subclass comprising alkalinehydro-' lyzable salts oi. an alkali metal and a weak acid. By the termsalts of an alkali metal and a weak acid I intend to include so-calledcomplex" salts .in which one or more alkali metals and one or more weakacid radicals are present, andsuch complex salts may contain in additiona metal other than an alkali metal or acid radicals other than weak acidradicals. An example of such a complex alkaline hydrolyzable salt of analkali metal and a weak acid is magnesium ammonium citrate, and this issuch a salt notwithstanding the presence of the non-alkali metalmagnesium. by the term weak acid" in this specification and the annexedclaims, I intend to include only acids whose dissociation constants areequal to or less than the dissociation constant of pyr phosphoric acid.For example, hydrochloric, sulphuric and nitric acids would not beweakacids" for the purpose of this class, since their dissociationconstants are greater than that of pyrophosphoric acid. Examples ofalkaline hydrolyzable salts of an alkali metal and a weak acid aretrisodium phosphate, disodium phosphate, trisodium arse-- nate, disodiumarsenate, trisodium citrate, tripotassium citrate, ammonium sodiumphosphate, sodium borophosphate, sodium glycerophosphate, magnesiumammonium citrate, sodium pyropbosphate, sodium metasilicate, sodiumsesquisilicate, sodium carbonate, sodium bicarbonate, sodiumsesquicarbonate, potassium chromate, potassium tartrate, potassiumborotartrate, sodium tetraborate, and potassium oxalate. Other membersof this class will be apparent to persons skilled in the art.

An example of a suitable coating solution using trisodiumorthophosphate, a member oi this class, V

is as. follows:

This solution is of a deep blue color and has a pH value of 6.11. I! theterricyanide is replaced in whole or in part by ferrocyanide in theabove .example, theamount oi trisodium orthophosphate should ordinarilybe increased. It will be apparent to persons skilledin the art that ifan alkali metal ion is present in a coating solution containing alight-sensitive ferric salt and a fer rocyanide, the incorporation ofanyweak acid into such a solution to thereby form with the alkali metalan alkali metal salt which hydrolyzes alkaline would be within the scopeof this invention as equivalent to the use ab initio of an alkalinehydrolyzable salt of an alkali metal and a weak acid Of the above classwhich I have defined as alkaline hydrolyzable salts of an alkali metaland a weak acid, I prefer to use a further subclass comprising alkalinehydrolyzable salts of an alkali metal and a weak polyvalent acid. By theterm "salts of an alkali metal and a weak polyvalent acid I intend toinclude any of the socalled complex salts in which one or more alkalimetals and one or more weak polyvalent acid radicals are present whetheror not such salts contain in addition metals other than alkali metals oracid radicals other than weak polyvalent acid radicals. By the term"weak polyvalent acids" I intend to include only polyvalent acids whosedissociation constants are equal to or less than the dissociationconstant of pyrophosphoric acid. For example, sulphuric acid is apolyvalent acid, the alkali metal salts, of which would not fall withinthis class, since its dissociation constant is greater than that of.p'yrophosphoric acid.

All of the alkaline hydrolyzable salts enumerated above as examples ofsalts of an alkali metal and a weak acid, happen to also besuch salts ofweak polyvalent acids,-and the said chemicals are here again, referredto as examples of this subclass. Other members of this class will beapparent to persons skilled in the art. The coating solution oftrisodium orthophosphate given above will also serve as an example of asuitable coating solution under this class.

Ifan alkali metal ion is present in a coating solution containing alight-sensitive ferric salt and a fcrrocyanide, the incorporation intosuch a solution of any weak polyvalent acid to thereby form an alkalimetal salt which hydrolyzes alkaline, would be within the scope of thisinvention as equivalent to the use ab initio of an alkaline hydrolyzablesalt of an alkali metal and a weak polyvalent acid.

Of the subclass which I have defined as alkaline hydrolyzable salts ofan alkali metal and a weak poiyvalent acid, I prefer to use a member ofa further subclass comprising alkaline hydrolyzable salts of an alkalimetal and a trivalent acid. By the term salts of an alkali metal and atrivalent acid I intend to include any of the so-called complex" saltsin which one or more alkali metals and one or more trivalent acidradicals are present whether or not such salts contain in additionmetals other than alkali metals or acid radicals other than trivalentacid radicals. Exam-' ples of members of this class are trisodiumphosphate, disodium phosphate, trisodium arsenate, disodium arsenate,trisodium citrate, tripota'ssium citrate, ammonium sodium, phosphate,sodium borophosphate, sodium 'glycerophosphate, and magnesium ammoniumcitrate. Other members of this class will be apparent to persons skilledin the art. j Y t An example of a suitable coating solution usingtripotassium citrate, a member of this class, is as follows:

200 grams of ferric ammonium oxalate and 25.5 grams of tripotassiumcitrate dissolved in 500 cc. of .water, and to this solution add: 30grams of potassium ferrocyanide and 25 grams of potassium ferricyanide'dissolved in 500 cc. of water.

This solution is of a deep blue color and has a pH value of 5.22. If theferricyanide is replaced in whole or in part by ferrocyanide in theabove example, the amount of tripotassium citrate should ordinarily. beincreased. The use of a trivalent acid to form an alkaline hydrolyzablealkali metal salt with alkali metal ions present in the coating solutionis within the scope of this invention as equivalent to theuse ab initioof an alkaline hydrolyzable salt of an alkali metal anda trivalent acid.

Of the subclass which I have defined as alkaline hydrolyzable salts ofan alkali metal and a weak polyvalent acid, I next prefer to use amember of the subclass comprising alkaline hydrolyzable salts of analkali metal and.a weak divalent acid. By the term "salts ofan alkalimetal and a weak divalent acid I intend to include any of the so-calledcomplex salts in which one or more alkali metals and one or more weakdivalent acid radicals are present whether or not such salts contain inaddition metals other than alkalimetals or acid radicals other than weakdivalent acid radicals- By the term weak divalent acid I intend toinclude only divalent acids whose dissociation constants are equaleto orless than the dissociation constant of pyrophosphoric acid. For example,sulphuric acid is a divalent acid, the alkali metal salts of which 200grams of'ferric ammonium oxalate and 16.75 grams of sodium metasilicatedissolved in 500cc. of water, and to this solution add:

30 grams ofpotassium ferrocyanideand 25 in 500 .cc. of water.

This solution is of a deep blue co'lorand has a pH value of 6.81. If the'ferrocyanide isreplaced in whole'or in part by ferrocyanide in theabove example, the amount of sodium metasilicate shouldordinarilybeincreased. The use of a weak divalent acid to form analkaline hydrolyzable alkali metal salt with alkali metal ions presentin the coating solution is within the scope of this invention, asequivalentto the use ab initio of an alkaline hydrolyzable salt of anyalkali metal anda weak'divalent acid;

grams of potassium ferricyanide dissolved 'Of the class which I havedefined as alkaline hydrolyzable salts of an alkali metal and a weakpolyvalent acid, Imost prefer to use the subclass comprising. alkalinehydrolyzable alkali metal phosphates, and of these I prefer sodiumpyrophosphate. phates" I intend to include the so-called complexcompounds in which one or more alkali metals and one or more phosphateradicals are present whether or not such compounds contain in additionmetals other than alkali metals 'or acid radicals other thanprosphateradicals; liixamples of so-called complex compounds hereinincluded in the term alkaline hydrolyzable alkali metal phosphates aresodium borophosphate, sodium glycerophosphate, ammonium so diumphosphate, and magnesium ammonium phosphate. Other members'of thisclassare tri- By the term alkali metal phos-- 200 grams of ferricammonium oxalate and 20.0 grams of sodium pyrophosphate dissolved in 500cc. of water, and to this solution add:

grams ofpotassium ferrocyanide and 26 grams of potassium ferricyanidedissolved in 500 cc. of water.

This solution is of a deep blue color and has a pH value of 5.83. If theferricyanide is replaced in'whole or in part by ferrocyanide in theabove example, the amount of sodium pyrophosphate should ordinarily beincreased.

An example of a coating solution using sodium borophosphate, anothermember of thisclass. is as follows:

200 grams of ferric ammonium oxalate and 24.0 grams of sodiumborophosphate dissolved in 500 cc. of water,,and.to this solution add:

30 grams of potassium ferroc'yanide and V 25 grams of potassiumferricyanide dissolved in 500 cc. of water.v

This solution is 'of a deep blue-color and has a pH value of 5.61. Ifthe ferricyanideis replaced in whole or in part by ferrocyanide in theabove example, the'amount of sodium borophosphate should ordinarily beincreased.

' It will be ,apparent to one skilled in the art that'if alkali metalions are present in acoating solution containing a light-sensitiveferric salt and a ferrocyanide, the incorporation of phosphates or anyphosphoric acid intosuch asolution to thereby form an alkali metalphosphate which hydrolyzes alkaline would be within the scope of thisinvention as equivalent to use ab initio of an, alkali metal'phosnhatewhich hydrolyzes alkaline. g

Of the ,class which I have defined as alkaline hydrolyzable alkali metalcompounds, the subclass which 1 least prefer to use is the classcomprising 'the alkali metal hydroxides of which class sodium, potassiumand ammonium hydrox- -ides are representative member-another members ofthisclass willbe apparent to persons skilled in this art. Asuitablecoating solution using sodium hydroxide is as. follows:

This solutionis of deep blue color and has pH value of 6.78.

As hereinabove pointed out, these hydroxides if used in suiilcientquantities tend to bring the coating solution to neutral or alkaline. Asuitable coating solution; using monopotassium,

phosphate (a suitable alkali metal salt which hys,oos,7ss

drolyzes acid as hereinabovestated) in addition to sodium hydroxide isas follows:

200 grams of ferric ammonium oxalate, and

6.0 grams of sodium hydroxide. and 11.0 grams of monopotassium phosphatedissolved in 500 cc. of water, and to this solution add: '30 grams ofpotassium ferrocyanide and 25 grams of potassium ferricyanide dissolved.in 500 cc. of water.

This solution is of a deep blue color and has a pH-value of 5.88.

It will be apparent to persons skilled in this art that it is within thescope of this invention and the annexed claims to first coat the paperwith a solution containing, or to prepare a paper with a sizingcontaining, one or more alkaline hydrolyzable alkali metal compounds andthen to subsequently coat this paper with a solution containingalight-sensitive ferric salt and a ferrocyanide, since the two coatingswould combine to form the coating of this invention.

In this specification and the annexed claims when I refer to a coatingcontaining alkaline hydrhlyzable alkali metal compounds. light sensitiveiron salts in the ferric state. and ferrocyanides, I intendto include acoating solution containing, a coating'containing, and a paper coatedwith a coating containing the reaction products of said alkali metalcompounds, said light-sensitive ferrlcsalts and said ferrocyanidesbecause I do not know whether or not said chemicals retain theiroriginal identity after they are brought together in a coating solutionor a coating.

It will be understood that the use of ferricyanide may be entirelyeliminated in any of my coating solutions hereinabove described, andalso that the proportions of ferricyanide to ferrocyanide may-be variedat will as has been indicated. Generally speaking the more fen'icyanidepresent in relation to the ferrocyanide present, the slower is theprinting speed.

It'will also be understood that more than one of the alkalinehydrolyzable alkali metal compounds may be used in the same coating solution even though these compounds are selected from different subclassesas defined in this specification.

It will be further understood that chemicals other than alkali metalcompounds which hydrolyze alkaline, light-sensitive ferric salts,ferrocyanides, and ferricyanides may be incorporated in the coatingsolution or in the coating on the paper without departing from myinvention. 80 long as the water solution of the coating. is acid andcontains at least one alkaline hydrolyzable alkali metal compound, alight-sensitive ferric salt, and an alkali metal ferrocyanide, it isintended to be within the scope of the annexed claims, even if thecoating solution or the coating on the paper. contains other chemicalsin addition to those specified.

Suitable light-sensitive iron salts in the ferric state other thanferric ammonium oxalate. suitable alkali metal ferrocyanides other thanpo-. tassium ferrocyanide, and suitable ferricyanides other thanpotassium ferricyanide will be apparent to those skilled in the art,

The determination of the alkalinity, neutrality, or acidity ofhydrolyzation of the compounds and salts recited in this specificationand the determination of the pH values of coating solutions given inthis specification has been made ing to this invention can be accuratelydeter-' mined. By an acid coating solution I mean one which has a pHValue of less than '7 and which value is determined by said method or byone which is equivalently accurate. By an alkaline hydrolyzable compoundor salt I mean one the water solution of which has a pH value greaterthan 7 and which value is accurately determined by said method or itsequivalent.

Persons practicing this invention should guard against using appreciablequantities of a salt or compound which is extremely strong in itsreducing effect, since such a salt will tend to reduce thelight-sensitive iron salt before the paper is exposed to light and thusdestroy or deteriorate the light-sensitiveness of the coating. Sodiumsulphite is an example of such a salt.

In use the paper is sensitized by coating at least one surface thereofwith a coating solution made in accordance with this invention, and itis then dried and ready for use. Such a coatedpaper is dark blue-greenin color as distinguished from the lemon-yellow color of prior artpaper. The coating may be eifected by brushing; by spraying, by passingthe paper through a machine in contact with rollers which dip into abath of the solution or by any other suitable coating procedure. Thediagram is then superimposed upon the sensitized surface of the drypaper, is printed by exposure to'light, and the printed or exposed paperis passed through a water bath after which it may be hung up to dry. Ifdesired, before this final drying the paper may be passed through anoxidizing bath, then through another water bath and then dried.

The acidity of the coating solution as a whole increases the speed ofreduction of the light-sensitive iron salt and thus an acid coatingsolution provides a much faster printing paper than a coating solutionwhich is neutral or alkaline.

Paper coated with the acid coatingsolutions of a light-sensitive ferricsalt and ferrocyanide made in accordance with my invention may be keptin sensitized condition without'spoilage for much longer periods of timebefore'exposure' than the papers of the prior art, and will produceperfectly clear and unfogged whites when subsequently printed. Theacidity of my coating solution causes the size in the paper to swell andthus to absorb the coating solution much more readily than neutral oralkaline solutions, and consequently a less concentrated coatingsolution is required to provide sufficient of the sensitized coatingupon the surface of the paper for a satisfactory deep blue background inthe final print, whereas in the use of neutral or alkaline solutions itis necessary to use solutions which are highly concentrated and whichrequire heating to keep the chemicals in solution before and duringcoatin' It is to be understood that my invention is not dependent uponthe theories which I have set forth as descriptive of the chemicalactions involved nor dependent upon the soundness or accuracy oftheoretical statements so advanced, such theories or'theoreticalstatements are merely what I believe to be correct and I intend to claimeverything inherent in my invention which is novel in view of the priorart. v I

1. A sen tized blue print paper coating con-- taining an a linehydrolyzable alkali metal compound, a light-sensitive ironsalt in theferric state and analkali metal ferrocyanide, the proportions of saidalkali metal compound to the other ingredients of'the coating being suchthat a water solution of; said coating has apH value of less than 7 butnot suillciently low a permit the formation of an insoluble bluecompound prior to exposure to light.

2. A sensitized blue print paper coating containing an alkalinehydrolyzable salt of an alkali metal and a weak acid, a light-sensitiveiron salt in the ferric state and an alkali metal ferrocyanide, theproportions of said alkaline hydrolyzable salt to the other ingredientsof the coating being such thata water solution of said coating has a pHvalue of less than 7 but not sufficiently low to permit the formation ofan insoluble blue compound prior to exposure to light. Y

3. A sensitized blue print paper coating containing an alkalinehydrolyzable salt of an alkali metal and a weak polyvalent acid, alight-sensitive iron salt in the ferric state and an alkali metalferrocyanide, the proportions of said alkaline.

hydrolyzable salt to the other ingredients of the coating being suchthat a water solution of said coating has a pH value of less than 7 butnot sufficiently low to permit the formation of an insoluble bluecompound prior to exposure to light.

4. A sensitized blue print paper coating containing an alkalinehydrolyzable salt of an alkali metal and a weak divalent acid, alight-sensitive iron salt in the ferric state and an alkali metalferrocyanide, the proportions of said alkaline hydrolyzable salt to theother ingredients of the coating being such that a water solution ofsaid coating has a pH value of less than 7 but not sufllciently low topermit the formation of an insoluble blue compound prior to exposure toli ht.

5. A sensitized blue print paper coating containing an alkalinehydrolyzable salt oi' an alkali metal and a weak trivalent acid, alightsensitive iron salt in the ferric state and an alkali metalferrocyanide, the .proportions of said alkaline hydrolyzable salt to theother ingredients of the coating being such that a water solution ofsaid coating has a pH value of less than 7 but not sufficiently low topermit the formation of an insoluble bluecompound prior to exposure tolight.

6. A sensitized blue print paper coating containing an alkalinehydrolyzable alkali metal phosphate, a light-sensitive iron salt in theferric state and an alkali metal ferrocyanide, the proportions of saidalkali metal phosphate to the other ingredients of the coating beingsuch that a water solution of said coating has a pH value of less than 7but not sufficiently low to permit the formation of an insoluble bluecompound prior to exposure to light.

'7. 'A sensitized blue print paper coating containing an alkali metalhydroxide, a light-sensitive iron salt in the ferric state and an alkalimetalferrocyanide, the proportions of said alkali metal hydroxide to theother ingredients of the coating being such that a water solution ofsaid coating has a pH value of less than 7 but not sufficiently low topermit the formation of an {insoluble blue compound prior to exposure to8. A sensitized blue print paper coating containing sodium metasilicate,a light-sensitive iron salt in the ferric state and an alkali metalferrocyanide, the proportions 01' said sodium metasilicate to the otheringredients oi the coating being such that a water solution of saidcoating in: being such that a water solution of said coatinghasapHvalueoilessthan'lbut notsuiriciently low to permit the formation of aninsoluble blue compound prior to exposure to light.

10. A sensitized blue print paper coating containing sodium hydroxide,alight-sensitive iron salt in the ferric state and an alkali metalterrocyanide, the proportions of said sodium hydroxide to the otheringredients of the coating being such that a water solution of saidcoating has a pH value 01' less than 7 but not su'fliciently low topermit the formation of an insoluble blue compound prior to exposure tolight.

ALGER- P. REYNOLDS.

